1. Field of the Invention
The present invention relates to a numerical control apparatus that has function to perform motion control according to a machine coordinate system.
2. Description of the Related Art
In numerical control apparatuses for controlling machines such as machine tools, motion control is carried out on the basis of positions in a machine coordinate system which is set in advance for a machine when positioning a workpiece as an object to be machined or otherwise worked at a fixed position or positioning a moving part of a machine at a fixed position for tool exchange. A machine coordinate system is set for a machine and is fixed with respect to the installation position of the machine.
On the other hand, when machining or otherwise working a workpiece, a coordinate system is set for the workpiece, and machining or the like is carried out on the basis of positions in the workpiece coordinate system. Machining programs for machining a workpiece is usually created on the basis of such a workpiece coordinate system and machining is carried out in accordance with positions in the workpiece coordinate system.
Feeding of a moving axis according to a workpiece coordinate system is carried out at a feed rate specified by a machining program or the like, but a feed rate in positioning on the basis of a machine coordinate system is a rapid traverse speed which is set by parameters and a fixed value.
In case of a movement command according to a machine coordinate system, a moving part of a machine is driven at a rapid traverse speed set in advance, resulting in a high-speed axis movement. However, a high-speed movement may not be suitable for some workpieces or some peripheral environment of a machine. Interference due to an overshoot may occur when a moving axis is being stopped, a machine may suffer vibrations due to high speed movement so that this vibration may adversely affect the machine and its peripheral devices. This situation is particularly problematic for machine tools that operate under multi-path system control. When cutting is in progress in one path while a movement command is issued on the basis of a machine coordinate system in other path, for example, a tool exchange command is issued and positioning is carried out on the basis of a machine coordinate system, in the other path, a high-speed axial movement will be carried out under a set rapid traverse speed up to a tool exchange position in the above “the other path”. This high-speed axial movement will cause vibration on the machine, and this vibration will adversely affect the ongoing cutting process in the above “one path”, thereby lowering the machining precision on the machined surface.